To perform an internal operation, a semiconductor memory device receives an external voltage such as a supply voltage VDD and a ground voltage VSS from outside, and generates an internal voltage to be used for the internal operation. The internal voltage of the memory device may include a core voltage VCORE supplied to a memory core region, a high voltage VPP used for driving a word line, and a back bias voltage VBB supplied as a bulk voltage of an NMOS transistor of the core region.
The core voltage VCORE may be generated by lowering the supply voltage VDD to a predetermined level. However, the high voltage VPP has a higher level than the supply voltage VDD inputted from outside, and the back bias voltage VBB has a lower level than the ground voltage VSS inputted from outside. Therefore, a charge pump circuit is required to supply charges for generating the high voltage VPP and the back bias voltage VBB.
An internal voltage used in a semiconductor memory device may include a negative word line voltage VNWL. The negative word line voltage VNWL is a voltage which is supplied to a word line in a state such as a precharge state in which the word line is not selected, and maintains a lower level than the ground voltage VSS to thereby reduce a leakage current without increasing the threshold voltage of a memory cell transistor.
When a leakage current occurs between a word line and a bit line (e.g., a leakage current due to a bridge between the word line and the bit line) in a state in which the negative word line voltage VNWL is applied to the word line, a potential difference between the word line and the bit line increases in proportion to the absolute value of the negative word line voltage VNWL. Furthermore, the threshold voltage of a memory cell becomes high at low temperature, and thus the potential difference between the word line and the bit line becomes large. Accordingly, a leakage current may increase.
Since the negative word line voltage VNWL is generated at a constant level regardless of temperature, the leakage current further increases at low temperature than at high temperature, thereby degrading a refresh characteristic.